The new UI has only the editor pane, whereas there used to be a preview to the right which was updated as you typed. I found the preview very helpful for ensuring I was using markup correctly.
I looked under preferences/interface but didn’t see an option.
Is it no longer a feature?
Ah, that’s what the A toggle does on the editor UI.
That “[Switches] to standard Markdown editor (Ctrl M)”, which is the editor with the preview pane. The A changes to M↓ (Mark Down) when you select it.
It looks like the new A “Rich Text” editor aims to combine the two, WYSIWYG style.
You, and if you have cookies enabled that setting will persist.
(I’m very much a “compose over here in plain text, see the result rendered over there” person - see also my wibblings about Typst. 
)
Turns out this is also relevant to using the dice roller: if you put in a square bracket in the new WYSIWYG mode, it’ll be escaped with a backslash, and so the dice roller won’t spot it. So if you’re calling for a die roll, switch to the old markdown mode first.